1. Field of the Invention
The present invention relates to a heat fixing device and an electrophotographic apparatus having the heat fixing device. In particular, this invention is concerned with a heat fixing device that uses a PTC heating element, i.e., an element having a positive temperature coefficient of resistivity, as a heating element which applies heat energy to a recording material via a belt, and also concerned with an electrophotographic apparatus incorporating the heat fixing device.
2. Description of the Related Art
A fixing device of the type known as heat roller-type fixing device is widely used in various electrophotographic apparatus such as a copying machine or an optical printer. This fixing device comprises a heating roller that has a built-in heater and held at a predetermined temperature, and a pressurizing roller that is coated with an elastic layer, and pressed against the heating roller. These rollers cooperate with each other to nip therebetween a recording medium such as a paper sheet carrying unfixed toner image so as to thermally fix the toner image by applying pressure while conveying the recording medium.
According to this heat roller fixing method, since the heating roller has a large heat capacity, it takes much time (warm-up time) until the heating roller is heated up to a temperature high enough to fix the toner image. In addition, a large power is consumed for maintaining the temperature.
In addition, it is not easy to set the temperature and angle of a point at which the recording material is peeled off from the heating roller. Consequently, problems are caused such as adhesion of toner to the heating roller (referred to as "offset") or clinging of the recording paper sheet around the heating roller.
Various belt-type fixing devices have been proposed to overcome these problems. One of such the belt-type fixing devices employs a belt which is moved in contact with a heating unit, a pressing member which presses a recording medium into close contact with the side of the belt opposite to the side contacting the belt, the heating unit being moved together with the belt, whereby heat energy is applied from the heating unit to the recording material via the belt.
The belt-type fixing device offers a higher heat efficiency than the heat roller fixing technique, and reduces a warm-up time and a power consumption. After heated and melted, toner on paper is cooled down sufficiently and fixed to the paper. The heating unit is then separated from the belt.
A thick-film heater, a halogen lamp, a PTC heating element or the like has been proposed for use as the heating unit employed in the belt-type fixing technique. Japanese Patent Laid-Open No. 63-313182 uses a thick-film heater that is made by coating an alumina substrate with a linear or a belt-like electrical resistive material made of Ta2N or the like and that has a low heat capacity. The surface of a heating unit has a Ta.sub.2 O.sub.5 layer that protects the heating unit from the rubbing against the belt. A technique using the radiant heat of, for example, a halogen lamp has been disclosed in U.S. Pat. No. 3,811,828 or Japanese Patent Laid-Open No. 3-25475. Japanese Patent Laid-Open No. 2-158782 discloses a technique in which a PTC heating element is employed as a heating unit. The PTC heating element is united with a linear heating plate, which is made of a metal such as alumminium or copper, or a ceramic such as alumina, by means of an adhesive.
Among the foregoing heating units, a heating unit based on a PTC heating element facilitates the detection and control of its temperature, as compared to a heating unit using a thick-film heater or a halogen lamp, and enables easier reduction of maximum power consumption than any other heating unit. This is because the PTC heating element has such a self-temperature control ability that, when the own-temperature reaches a temperature inherent to the material of the PTC heating element (Curie point), the resistance increases drastically to suppress heating.
When a PTC heating element is employed in a belt-type heat fixing device, a pinch effect specific to the PTC heating element (a phenomenon that when a temperature distribution varies in the PTC heating element, current is restricted so that the heating value decreases eventually) makes it impossible to fully utilize the heating value of the PTC heating element. Even when the temperature on a radiant surface of the PTC heating element decreases due to heat absorption by belt, if the temperatures of the other portions than the radiant surface reach the Curie temperature, the current value decreases with the resistance held high, failing to raise the temperature of the radiant surface.
When a supporting material for the PTC heating element deforms with heat, a gap is created between the radiant surface of the heating unit and the belt. Heat is not therefore supplied to the belt effectively, or the contact between the PTC heating element and electrodes becomes imperfect. This causes an arc discharge, resulting in breakdown of the PTC heating element.
In a belt-type heat fixing device using a PTC heating element, it is necessary that, even when the supporting member for the PTC heating element deforms by heat, good electrical contact is maintained between the PTC heating element and electrodes, as well as good thermal contact between the radiant surface of the heating unit and the belt. It is also necessary that the temperature distribution throughout the PTC heating element is as uniform as possible, and that the temperature of the entire PTC heating element is sensitive and responsive to a change in the temperature of the belt.
Japanese Patent Laid-Open No. 2-158782 discloses such a structure that a thinned PTC heating element is united with a linear heating plate, made of a metal such as aluminum or copper, or a ceramic such as alumina, by means of an adhesive. The temperature distribution in the PTC heating element is thus made uniform. However, no consideration has been given to the imperfect electric contact between the PTC heating element and the electrodes, and the imperfect thermal contact between the heating unit and the belt, which result from the thermal deformation of a supporting material for the heating unit. This brings about the aforesaid problems, causing impediment to the practical application of the prior art.